1. Field of the Invention
The present invention relates to a method for preparation of a moisture sensor, having a moisture sensitive film made of a polymer, which detects variation of humidity in the atmosphere by means of measurement of a variation in the impedance of the polymer film.
2. Description of the Prior Art
As a moisture sensor wherein an impedance varies depending upon a variation of humidity in the atmosphere, there have been, for example, a moisture sensor having a sintered body of metal oxides such as tin oxide (SnO.sub.2), or a metal oxide film; a moisture sensor having a hydrophilic polymer film; a moisture sensor having an electrolyte salt such as lithium chloride (LiCl); and a moisture sensor having a hygroscopic resin or polymer film in which conductive particles or fibers such as carbon are dispersed.
While a moisture sensor containing a metal oxide film or a hydrophilic film has a wide moisture-sensitivity range, its resistance varies exponentially responding to relative humidity in the atmosphere. A moisture sensor having an electrolyte salt such as lithium chloride detects only humidity in a narrow range so that when it is allowed to stand in a high humidity atmosphere for a long period of time, the electrolyte salt therein is eluted or diluted resulting in deterioration of the moisture sensitive characteristic of the sensor, and accordingly it cannot be used for determination of high humidity. Also, a moisture sensor having a hygroscopic resin or the like, in which conductive particles or fibers are dispersed, cannot detect a humidity in a wide range because it exhibits a steep variation of the resistance thereof in a high humidity atmosphere, while it is not sensitive to low humidity.
On the other hand, a moisture sensor having, as a moisture sensitive film, a hydrophilic polymer film or a polyhydrolyte has been put into practical use because it is advantageous over the above-discussed sensors in that it is easily produced; it has excellent reproducibility and interchangeability; it operates in a wide moisture sensitive range; it has a high sensitivity due to wide variation of the resistance; and it responds rapidly. This moisture sensitive film is formed on a substrate by the use of a dipping technique, a spin casting method, or the like, in general. However, these techniques do not provide a moisture sensitive film of the desired pattern on the substrate. Also, a moisture sensitive polymer film formed by a wet etching technique using an etchant has a water resistance and a solvent resistance due to cross-linkage/polymerization and/or heat treatment employed by the etching process, so that a wet etching technique cannot be applied to the moisture sensitive polymer film. Thus, the moisture sensitive polymer film is usually formed into a pattern in such a manner that the portion of the polymer film to be connected to the lead wire is melted out by means of a soldering tool, or removed together with a masking tape which was placed on the substrate prior to the formation of the polymer film. These methods are undesirable as the moisture sensitive polymer film deteriorates and/or blisters at the edge portions due to the heat of the soldering tool. Moreover, these methods do not provide a compact, accurate and reproducible pattern; for instance, a pattern having a size of 1 mm or less is not obtainable.
Although a screen printing method is known as another pattern formation technique, it requires a polymer solution having a high viscosity which tends to change, resulting in nonuniform quality, and also cannot provide an accurate and reproducible pattern thereby making the formation of a highly fine pattern difficult. For these reasons, a screen printing method is not usually used for the formation of a moisture sensitive polymer film.
As described above, it is difficult to form a fine and accurately patterned moisture sensitive polymer film by the use of the conventional techniques, so that a moisture sensor having a fine and accurately patterned moisture sensitive polymer film has not yet been developed.
A technology to miniaturize moisture sensors and to compound a plurality of moisture sensors has recently been developed, and a field effect transistor (FET) moisture sensor in which a moisture sensor material is disposed on a gate of FET has been developed. Thus, a technology to form a moisture sensitive film having a fine and accurate pattern has become increasingly required.